When, for example, a machine tool includes a large movable component that is to be moved, and that cannot be driven by just a single motor, the same movement command is provided to a plurality of motors to perform tandem control in which the movement of the movable component on the same axis of movement is driven and controlled by a plurality of motors.
JP 2003-079180 A discloses a technique of a tandem control electric motor control device that drives one movable component using a main electric motor and an auxiliary electric motor, the tandem control electric motor control device including a torque arbitrator for performing low-pass filtering on a difference between a torque command for a main control device and a torque command for an auxiliary control device to correct the torque command for the auxiliary control device. FIG. 6 is a block diagram illustrating main components for the tandem control disclosed in JP 2003-079180 A. Referring to FIG. 6, a position controller (not shown) calculates a speed command for an electric motor based on a common position command for controlling the position of the movable component and position feedback information detected by a position detector 4.
Speed controllers 11 and 21 respectively calculate torque commands Tm and Ts by performing control such as PI control based on the speed command received from the position controller and speed feedback information detected by speed detectors 15 and 25. A torque arbitrator 40 performs low-pass filtering on a difference between the torque command Tm for a main control device and the torque command Ts for an auxiliary control device, and adds the result to the torque command for the auxiliary control device. Current controllers 12 and 22 calculate voltage commands based on the torque commands Tm and Ts and the current feedback information. The current feedback information is not described here.
Servo amplifiers 13 and 23 output driving currents for driving electric motors 14 and 24 based on the voltage commands output from the current controllers 12 and 22, and drive the electric motors 14 and 24, which move a movable component 3 through power transmission mechanisms 16 and 26.
As described above, position, speed, and current loop control for the plurality of electric motors 14 and 24 is performed based on a common position command, and the movable component is driven by a combined torque output from the electric motors 14 and 24.
JP 2010-172054 A discloses a technique of two tandem control systems, torque tandem control and position tandem control, including a corrector for applying a preload torque value to torque commands in order to minimize backlash between two motors. FIG. 7 is a block diagram illustrating main components of a structure based on the tandem control disclosed in JP 2003-079180 A, and including a corrector for applying a preload torque value disclosed in JP 2010-172054 A. Referring to FIG. 7, an amount of preload outputter 70 adds a preload torque that assumes a constant value, to the torque command Tm for the main control device, and adds the preload torque, with the polarity inverted, to the torque command Ts for the auxiliary control device.
However, the technique of applying a preload torque to two motors disclosed in JP 2010-172054 A cannot be applied to the tandem control electric motor control device having the torque arbitrator disclosed in JP 2003-079180 A because, if applied, as illustrated in FIG. 7, a torque arbitration value is generated when a preload torque is applied or removed, and displacement of the movable component occurs. The mechanics of how displacement occurs when a preload torque is applied or removed will be described below. The speed controllers 11 and 21 output torque commands Tm and Ts, each for causing an amount of speed deviation to become zero based on the speed feedback information of the speed detector 15 or 25. At this time, if the torque arbitrator 40 is ignored, because the torque commands Tm and Ts are outputs that assume the same value with different polarities, the combined torque output from the electric motors is zero, and no displacement of the movable component occurs. However, actually, because the torque arbitrator 40 causes the torque command Ts for the auxiliary electric motor to assume a value different from the torque command Tm for the main electric motor, the combined torque output from the electric motors is not zero, and displacement of the movable component occurs.
As a result, when a power transmission mechanism, such as a reduction gear or a gear, of a machine has large backlash or torsional moment, because an increase in gains of speed and position control loops oscillates electric motors and causes vibrations or unusual noise, gain should be lowered, which degrades follow-up performance. When the backlash or torsional moment of a reduction gear or a gear is structurally reduced, the rigidity of the transmission mechanism should be increased, and the gear should be of a higher accuracy (grade), which increases the costs.
In the tandem control electric motor control device having a corrector for applying a preload torque value disclosed in JP 2010-172054 A, when the shaft structure requires a holding torque under the influence of gravity, because the holding torque and the preload torque for minimizing backlash are added to a torque command, the electric motors generate greater heat. As a result, greater thermal displacement shifts the center of the shaft, resulting in deterioration in machining accuracy, or the machine is stopped in response to an overload alarm for protecting the electric motors, resulting in decreased machining efficiency.
Although the problems can be solved by water cooling the electric motors or the machine as a method for minimizing such thermal displacement caused by heat generation, this approach increases the costs. Further, although the problems can be solved by employing electric motors having greater continuous rated power as a method for minimizing heat generation of the electric motors, the increase in capacity of the electric motors increases the costs, and because the volume of the electric motors is increased, this approach upsizes the machine and reduces the design flexibility of the machine.